There is a substantial need to identify biomarkers of risk for breast cancer. An in-depth quantitative proteomics approach was applied to the analysis of plasmas that were collected prior to a diagnosis of breast cancer in search for candidate markers of risk for this disease. The samples were obtained from the Women's Health Initiative (WHI) cohort and consisted of women diagnosed with breast cancer within seven years of blood collection and controls matched for age, self-reported ethnicity, hysterectomy status and enrollment date. In parallel studies proteomic profiling was applied to blood specimens obtained at baseline and following one year of hormone therapy (HT) with conjugated equine estrogen (CEE) or CEE/MPA (medroxyprogesterone acetate). Extensive proteomic analyses identified a large subset of circulating proteins that were affected by HRT, and has also yielded a set of breast cancer risk marker candidates that merit additional validation studies. Interestingly some of the risk candidates were also affected by HRT and thus may contribute to elucidation of breast cancer risk associated with CEE/MPA therapy. In aim 1, we propose to conduct a confirmation study of risk markers identified using an independent set of WHI participants from the WHI hormone therapy trials who developed breast cancer and matched controls. Of the 14 candidates to be subjected to confirmation studies, eight have ELISAs available that would allow their assay. The remainder of the candidates would be subjected to confirmation using Multiple Reaction Monitoring mass spectrometry. A second aim consists of evaluating the identified risk markers as mediators of hormone therapy effects on breast cancer. To that effect plasmas collected at baseline and at 1-year of HT in the CEE and CEE/MPA trials will be utilized to determine changes in concentration of risk marker candidates in cases and in matched controls. The proposed project has the potential to contribute clinically relevant breast cancer biomarkers to identify women at increased risk and to clarify breast cancer risk associated with postmenopausal hormone therapy.